The present invention relates to a tools supporting and heating device for tools like printing plates, used for hot embossing and/or diecutting and hot pressure transfer of portions of metallic films, mainly onto a paper or cardboard substrate.
Such operations are carried out for example in a machine including a platen press, in which a cardboard sheet is introduced to be printed with related print motifs issued from a usual metallized foil or film conveyed between this sheet and the heated upper platen. The pressure needed for transferring the metallized film on the cardboard sheet is controlled by the lower movable beam of the platen press. This movable beam is usually equipped with a stamping die, to which the counterparts of each plate-shaped tool of the upper beam are secured. These tools are usually defined for the one skilled in the art with the term of printing plates. Thus, in a recurring vertical movement, the lower beam is pressing the counterparts against the related printing plates, and sandwiching the cardboard sheet above which the metallized foil is arranged. The foil is thus in direct contact with the plate heated through the upper beam. The upper beam enables diecutting and transferring the portion of the metallized foil, corresponding to the printing plate imprint, on the cardboard sheet. Once the transfer has been carried out, the lower beam comes down again and the printed cardboard sheet is removed from the platen press so that the press is free again to receive a new sheet to be stamped. In the meantime, the stamping foil is unrolled so that a new blank surface is connected with the printing plates. The diecutting and hot embossing process can then be repeated.
To ensure that the printing plates are set according to various needs, a relatively thick plate, provided with a plurality of evenly distributed apertures is already in use. Such plates are commonly known as honeycomb chase. They are directly secured to the heating surface of the upper beam. The securing of the printing plates on the honeycomb chase is carried out with fastening clamps which have one end which grasps the edges of the printing plate and another end which is slipped into and tightened in the apertures by a clamping pin and an eccentric, for example. Such securing means are described in more detail in patent CH691361.
The heating of the printing plates is thus realized through the honeycomb chase, which is itself directly associated with the upper heating platen. That platen is heavy and massive, which enables handling strong pressures generated by the lower movable platen at the time of the stamping of the metallized foil and even sometimes at the time of a simultaneous sheet embossing operation. The stamping and embossing forces vary according to the whole surface of the patterns to be stamped and can typically range from 1 to 5 MN, for surfaces of worked sheets of about one square meter. The device that enables heating the honeycomb chase and consequently the secured printing plates is located inside the upper heating platen.
Such a platen usually includes a massive block, interdependent from the machine frame. At least one supporting plate is arranged against the lower surface of the block. A plurality of parallel pipes are machined in the thickness of the block, enabling the fitting of about twenty electric heaters. This supporting plate is furthermore divided into ten areas, so that the heaters located in each area can be independently operated. To that end, there is an electric supply network inside the upper beam, and it connects each heaters group to an exterior power input. To cause the temperature of the printing plates to register to an optimum value, usually ranging between 50° C. and 180° C., the electric board is equipped with a thermostatic regulation device connected to a plurality of temperature sensors. The sensors are usually located the closer to the honeycomb chase and distributed according to areas related to the various groups of heaters.
Patent FR2'639'005 refers to a hot gilding device similar to the abovementioned one. The heating device of one of the platens comprises six heating units which are interdependent and are separated the one from the other by spaces of about one millimeter. Each heating unit involves a stacking of various plates. The honeycomb chase enabling the later securing of the plates is made of an upper plate with a plurality of bored holes. Under that plate, a copper plate is acting as a heat dispatcher. Another plate milled with grooves and provided with the heating resistances is located underneath the latter. This set of plates finally lays on a last one comprising compact plastic leaves in alternation with alveolate leaves. This last plate constitutes a thermal insulation avoiding excess heat dispersion to the rest of the platen.
Such heating devices have many drawbacks that do not enable capacities improvement of these machines and that make them also not really polyvalent. Among these drawbacks, one will mainly notice the huge thermal inertia of several massive parts of these heating devices which decreases the machine capacities when one needs a quick adaptation to new temperature data. It can be the case, during a same stamping work, when a batch of cardboard sheets is not any more at the same temperature as the preceding batch. The reasons for such a difference of temperature between these two sheets batches is directly related to their storage area, where ambient temperatures were unequal, or is due to a rate increase of the machine. When processing with cardboard sheets at lower temperature, it will be necessary to compensate for the calorific loss of the printing plates coming in contact with these sheets within the shortest delays. However the thermal inertia of all units used in the known heating devices can require not less than ten minutes before the temperature sensors can register the temperature variation. The reaction time for correcting such sudden temperature variations is thus very long compared to the production rate, which can be about 4000 even 7000 sheets per hour.
Another drawback is that the fitting of known heating devices produces an important heat loss spreading in the important mass of the numerous plates, frames and other metal parts connected to the printing plates. This heat loss results in an excessive energy consumption compared to the energy just needed for the printing plates to be at their working temperature, which means a relative low output for said devices, inversely proportional to the energy consumption costs.
Another drawback of the devices is the required pre-heating times before they are operational. Pre-heating times can sometimes be about several hours which prevents any use of the machine. Moreover, they depend on several variable factors, namely on the initial temperature of the plate, on the working temperature of the printing plates, on the conductivity and the mass of materials used. Inversely, the thermal inertia of these materials prevents the machine from fast cooling and thus makes any handling more complicated, like the disassembly of the printing plates followed by the preparation for other work, as long as the temperature has not reached a suitable level.
Another drawback is that the various assembly parts connected to the heating device have to deal with dilatations and other physical constraints. These dilatations generate on one hand mechanical tensions and, on the other hand, important size changes must be taken into account at the time of the cold positioning of the printing plates for hot processing.
Another drawback is the required sorting of the heating areas that cannot be reduced or removed. In case only one printing plate infringes on a small portion of an adjacent heating area, it would nevertheless be necessary to control the heating of this whole adjacent area to ensure the temperature homogeneity of the printing plate. This homogenization is indeed necessary to ensure a right transfer on the whole surface of the printing plate.
Another drawback is the difficulty for current heating systems to regulate their temperature. As the heating areas have relatively rough surfaces, it is generally difficult to obtain a satisfactory temperature regulation of the areas located at the edge of the honeycomb chase. Indeed, these peripheral areas are subject to a temperature gradient showing a temperature loss of the printing plate as soon as the edge of the heating plate is reached. This loss is produced either naturally by surrounding conditions, where the ambient air is at a quite lower temperature than the one of the printing plates, or artificially by a blower located upstream of the platen press, used to facilitate the stripping of the rest of the metallized foil, once the latter is stamped on the cardboard sheet. Thus, if these areas are located near-by the periphery of the heating plate, their temperature can never be homogeneous. The result will be a real loss of quality of the transfer of the metallized foil, causing even the appearance of some defects on said portions.
Another drawback is that heating systems like these are not easy to repair and maintain. The main units are subject to breakdowns in electric resistances and temperature sensors. However, if one of those parts should be defective, it would then not be possible any more to use the related heating area and it could in fact paralyse the whole machine if one, or several printing plates, would stay, even partially, in this area.
Another drawback is that an important infrastructure is needed in the platen to heat the printing plates. However, all mechanical and electric embodiments do not enable in such a case the convertibility of that kind of machine into one intended for the cardboard sheets cutting. The cutting stations of a packaging production line are nevertheless, excepted for some modifications, identical to the platen presses of the invention. However, to carry out such a conversion, it is necessary to remove the honeycomb chase from the platens, the printing plates and the other specific tools in order to replace them by suitable tools such as a cutting die, provided with cutting rules and a cutting plate acting as support and counterpart. Since these transformations require sometimes heavy handling, the machine must be stopped and is thus not productive during that time.